Remarkable improvements have been obtained recently in terms of fluorinated ion-exchange membranes, and have made it possible to develop methods for electrolysing sodium chloride solutions by means of ion-exchange membranes. This technique makes it possible to produce hydrogen and sodium hydroxide in the cathode compartment, and chlorine in the anode compartment of a brine electrolysis cell.
In order to reduce energy consumption, it has been proposed in patent application JP 52124496 to use an oxygen-reduction electrode as cathode, and to introduce a gas containing oxygen into the cathode compartment in order to prevent the release of hydrogen, and to reduce the electrolysis voltage significantly.
In theory, it is possible to reduce the electrolysis voltage by 1.23 V by using the cathode reaction with supply of oxygen represented by (1) instead of the cathode reaction without supply of oxygen represented by (2): EQU 2H.sub.2 O+O.sub.2 +4e.sup.- .fwdarw.4OH.sup.- (1)
E=+0.40 V (relative to a standard hydrogen electrode). EQU 4H.sub.2 O+4e.sup.- .fwdarw.2H.sub.2 +4OH.sup.- (2)
E=-0.83 V (relative to a standard hydrogen electrode).
In general, a conventional membrane electrolysis cell employing the gas electrode technology comprises a gas electrode which is placed in the cathode compartment of the electrolysis cell in order to divide this compartment into a solution compartment, on the ion-exchange membrane side, and a gas compartment on the opposite side. The gas electrode is usually obtained by moulding a mixture of a hydrophobic substance, such as a polytetrafluoroethylene resin (hereafter referred to as PTFE), and a catalyst or support catalyst, so that it has hydrophobic properties preventing liquids from passing through. However, a gas electrode of this type progressively loses its hydrophobic properties when it is exposed to a high temperature of the order of 90.degree. C., and to an aqueous solution of sodium hydroxide having a high concentration of about 32% or more by mass during long-term electrolysis. For this reason, the liquid present in the solution compartment tends to penetrate the gas compartment. Further, because the gas electrode consists of a mixture which principally comprises a material containing carbon and a resin, it is mechanically fragile and tends to crack. These drawbacks have prevented the practical use of a gas electrode of this type for the electrolysis of a brine.
An electrolysis cell configuration of this type is described in patent application FR 2 711 675 (page 2, line 13 to page 3 line 7 and FIG. 1).
In order to resolve the drawbacks mentioned above, it has been proposed in patent JP-B-61-6155 to combine a gas cathode and an ion-exchange membrane into a single integral structure, that is to say a cell of the integral gas electrode/ion-exchange membrane type without division of the cathode compartment.
Although the problems of mechanical fragility have thus been solved, this type of cell configuration nevertheless has drawbacks such as, in particular, changing the membrane and the cathode.
If the water requirement is calculated for a membrane electrolysis cell comprising a cathode consisting of platinized carbon formed with PTFE on a silvered nickel grid, it is found that the electrochemical reaction taking place at the cathode--reaction (1)--consumes 2 mol of water per 4 mol of sodium hydroxide produced, i.e. 0.5 mol of water for one mole of sodium hydroxide.
The sodium hydroxide which is produced must have a strength between 30 and 35%, or else the current efficiency will be reduced by increasing the migration of the hydroxyl ions back through the membrane, and the membrane will be physically degraded. These specifications are given by chlorine/sodium hydroxide membrane manufacturers and are valid for all types of membranes. This involves the addition of water to dilute the sodium hydroxide which is produced, 4.5 mol of water per mole of sodium hydroxide (to obtain 33% strength sodium hydroxide).
The electro-osmotic flux through the membrane supplies 3.5 mol of water per mole of Na+ in the cathode compartment, when the NaCl concentration in the anode compartment is 220 g/l.
0.5+4.5=5 mol of water are therefore consumed for one mole of sodium hydroxide. 3.5 mol of water are therefore added per mol of sodium hydroxide, i.e. a deficit of 1.5 mol of water per mol of sodium hydroxide under conventional operating conditions.
It has been proposed, in patent application EP 686 709, to add this "missing" water in the form of droplets of water in suspension in the oxygen (mist). However, the cathode is a hydrophobic electrode, because of the PTFE used as a binder, which is relatively compact. Further, the oxygen is in contact with the rear face of the electrode. Not all of the water provided by the gas will pass through the cathode to the membrane (in countercurrent with the sodium hydroxide which is produced) and will therefore serve to dilute the sodium hydroxide at the rear of the electrode, and not at the membrane/cathode interface. The result of this is that the amount of water available in contact with the membrane will be at best 3.5 mol of water per mole of sodium hydroxide, assuming that the water needed for the electrochemical reaction is supplied by the gas. This means that the sodium hydroxide concentration at the membrane/cathode interface will be greater than 40/(3.5.times.18+40).times.100=38.8%. Under these conditions, the current efficiency is poor and the lifetime of the membrane is shortened.